In general, in a liquid crystal display device that performs color display, one pixel is divided into three sub pixels of a red pixel, a green pixel, and a blue pixel, the red pixel being provided with a color filter that transmits red light, the green pixel being provided with a color filter that transmits green light, the blue pixel being provided with a color filter that transmits blue light. While color display is possible by use of the color filters provided in the three sub pixels, about two-thirds of backlight light applied to a liquid crystal panel is absorbed in the color filters. Hence a liquid crystal display device employing a color filter system has a problem of low efficiency in light utilization. Attention has thus been focused on a liquid crystal display device employing the field sequential system which performs color display without using color filters.
In a typical liquid crystal display device employing the field sequential system, one frame period, which is a display period for one screen, is divided into three fields. Although field is also referred to as sub frame, the term “field” will be used throughout the following description. For example, one frame period is divided into: a field (red field) that displays a red screen based on a red component of an input image signal; a field (green field) that displays a green screen based on a green component of the input image signal; and a field (blue field) that displays a blue screen based on a blue component of the input image signal. By displaying the primary colors one by one as described above, a color image is displayed on the liquid crystal panel. Since the color image is displayed in this manner, the color filters are not required in the liquid crystal display device employing the field sequential system. Accordingly, the efficiency in light utilization of the liquid crystal display device employing the field sequential system is about three times as high as that of the liquid crystal display device employing the color filter system. The liquid crystal display device employing the field sequential system is thus suited for high luminance and lower power consumption.
It should be noted that, in this specification, a color specified by a combination of a data value of a red component, a data value of a green component, and a data value of a blue component (a combination of a data value of a red component, a data value of a green component, a data value of a blue component, and a data value of a white component in a case in which a field that displays a white is provided) while considering display order of colors in a frame is referred to as “order color” for the sake of convenience. For example, a color specified by “first field: R=128, second field: G=32, third field: B=255” is one order color. In this example, colors are displayed in the order of “red, green, blue” in each frame. A data value of a red component is 128, a data value of a green component is 32, and a data value of a blue component is 255. A data value is typically a gradation value.
Meanwhile, in the liquid crystal display device, an image is displayed by controlling a transmittance of each pixel with a voltage (liquid crystal application voltage). In this regard, it takes several milliseconds for the transmittance at a pixel to attain a target transmittance from the start of writing data (applying a voltage) into the pixel, as shown in FIG. 44. Hence in the liquid crystal display device employing the field sequential system, in each field, a backlight of the corresponding color is switched from an unlighted state to a lighted state after the liquid crystal has responded to some extent. Namely, in the liquid crystal display device employing the field sequential system, the backlight is turned on only in a part of the latter half of each field (for example, a period indicated by reference character T9 in FIG. 44).
Further, in the liquid crystal display device, a sufficient image quality may not be obtained, for example at the time of displaying a moving image, due to a low response speed of the liquid crystal. Then, as one of measures against the low response speed of the liquid crystal, a drive system called overdrive (overshooting drive) has conventionally been adopted. The overdrive is a drive system in which the liquid crystal panel is supplied with a drive voltage higher than a predetermined gradation voltage corresponding to a data value of an input image signal in the current frame or a drive voltage lower than a predetermined gradation voltage corresponding to a data value of an input image signal in the current frame in accordance with a combination of a data value of an input image signal in the preceding frame and a data value of an input image signal in the current frame. That is, the overdrive leads to correction of an input image signal that emphasizes (not a spatial change but) a temporal change in a data value. By adopting such an overdrive, in the liquid crystal display device employing the color filter system, the liquid crystal makes a response such that the transmittance at a pixel attains the target transmittance in each field.
It should be noted that, regarding the present invention, WO 2010/084619 A discloses an invention in which the overdrive is applied to the liquid crystal display device employing the field sequential system.